This invention relates to an electromagnetic filter whose frequency may be quickly and easily altered.
Electromagnetic filters may filter a stream of electromagnetic energy by separating certain frequencies from the stream and/or by adding certain frequencies to the stream. For example, an optical filter may subtract a band of frequencies from a beam of light containing multiple frequencies of light. Such filters may be used in communication networks transmitting information using beams of electromagnetic information.
A communication network transports information from a source to a destination. The source and destination may be in close proximity, such as in an office environment, or thousands of miles apart, such as in a long-distance telephone system. The information, which may be, for example, computer data, voice transmissions, or video programming, known as xe2x80x9ctrafficxe2x80x9d, usually enters and leaves a network at nodes, and is transported through the network via links and nodes. Nodes, sometimes termed offices, are devices or structures that direct traffic into, out of, and through the network. Links connect nodes and transmit data between nodes.
Modem communication networks may transmit information in digital form by light waves using links of optical fiber cable. Multiple wavelengths of light may be transmitted on one optical fiber line, each wavelength carrying a separate channel of information. One wavelength of light may carry 2.5 gigabits of information per second in one direction, and current optical fiber lines may carry 16 wavelengths at the same time. Data may be sent in two directions at the same time on one link. A network using optical fiber cable carrying multiple wavelengths is called a wavelength division multiplexed (xe2x80x9cWDMxe2x80x9d) optical network.
The wavelength and the frequency of electromagnetic radiation are related in a fixed manner; thus electromagnetic energy and filters for electromagnetic energy may be characterized using both measures interchangeably.
Specific wavelengths carried on an optical fiber line may be added to the line or dropped (i.e., removed) from the line using an add/drop filter. Such a filter accepts as an input an optical fiber line transmitting a beam of electromagnetic energy carrying multiple frequencies, including the xe2x80x9ctargetxe2x80x9d or xe2x80x9ctunedxe2x80x9d frequency for which the filter is tuned (the frequency at which the filter operates, or the frequency centered in the band of frequencies at which the filter operates). The filter selects the frequency for which the filter is tuned (the xe2x80x9cdrop frequencyxe2x80x9d) from the beam on the optical fiber line and provides two outputs to two optical fiber output lines. A first optical fiber output line receives the original beam from the input optical fiber line, with the tuned frequency removed, and a second optical fiber output line receives the drop frequency, separated from the original beam. The filter may accept as an input an optical fiber line carrying a frequency to be added (xe2x80x9cadd frequencyxe2x80x9d), which corresponds in frequency to the drop frequency. In such a case the first output line receives the original beam with the add frequency replacing the drop frequency. It is not necessary that the drop frequency exist in the original beam: the filter may be used to add a frequency, add a frequency and drop a frequency, or drop a frequency.
When used herein, a frequency may include a range of frequencies covering a bandwidth (a range of frequencies covering a portion of the electromagnetic spectrum). When used herein, a frequency or wavelength may refer to a beam, or a component of a beam, containing a frequency or a band of frequencies surrounding a certain frequency. A data signal or channel may be carried on a band of frequencies surrounding a certain frequency. A multiple frequency beam is a beam of electromagnetic energy containing different channels which use different frequencies.
The frequencies added and dropped from a line may carry data. Frequencies may be dropped because a node requires access to the portion of the data carried on the optical fiber beam. A frequency may be added after a node alters the information on the frequency, which was dropped, or if the frequency does not exist on the beam. A node may need to add, subtract, monitor or modify data on one or more frequencies on a beam carried on a fiber, and may need to add or drop more than one frequency. Typically, one filter is used for each frequency for which access is desired. Filters may be used to multiplex multiple frequencies of data onto one optical fiber line. Filters are used to selectively add (multiplex) or drop (demultiplex) frequencies from a fiber.
When used herein, xe2x80x9cmultiplexingxe2x80x9d may include demultiplexing, and xe2x80x9cmultiplexerxe2x80x9d may include a device having demultiplexing capabilities. A filter adding and/or removing a wavelength of light from a link may be termed a multiplexer, an add/drop filter, or an add/drop multiplexer (xe2x80x9cADMxe2x80x9d). At each node one ADM is required for add/drop capability for each of the multiple wavelengths that may be carried on an optical fiber cable.
One known network is organized as a mesh. FIG. 1 is a block diagram illustrating a simplified portion of a mesh network. Referring to FIG. 1, mesh network 300 comprises nodes (e.g. nodes 304, 306, 308 and 310) connected by links (e.g. links 305, 307, and 309) transmitting traffic between nodes. For example, nodes 304 and 306 are connected by, and may transmit traffic via, link 305. For clarity, not all nodes and links in FIG. 1 are identified with reference numerals. Each node in network 300 may access some or all of the frequencies carried by the links to which it is connected. An add/drop filter is required at a node if traffic is to be added or dropped from a link on a certain frequency.
Typically, a node may add, drop and reroute traffic which originates or terminates at that node in order to allow customers connecting to that node access to that traffic or to route traffic to other nodes. For example, a customer connecting to node 310 may transmit traffic to a customer connecting to node 304 via links 309 and 305 and node 306, using a certain frequency. In such a case, both nodes 310 and 304 require add/drop filters tuned to that frequency. At some point the traffic on the frequency may need to be rerouted to flow to node 308 rather than 304; in such a case node 306 requires an add/drop filter to be able to access the frequency and, using equipment such as a cross connect, route the frequency to link 307 and node 308.
Networks employing architectures other than mesh configurations are also known. Ring networks, for example, interconnect nodes, using links, in a circular fashion to form rings. Multiple rings may be interconnected to form a network.
FIG. 2 is a block diagram illustrating a simplified portion of a ring network. Referring to FIG. 2, network 330 includes nodes 332, 334, 336, 338 and 340. Nodes are connected by links 331, 333, 335, 337, and 339. Nodes may use add/drop filters to add or drop a frequency from a line. For example, node 332 may send data to node 336 using a frequency of 2xc2x71014 Hz via links 331 and 333 and node 334. Node 336 receives a beam of light on link 333 which contains multiple frequencies, including 2xc2x71014 Hz, and transmits most of those frequencies unaltered on to link 335. Node 336 also receives a beam of light containing multiple frequencies on link 335 and transmits most or all of those frequencies unaltered on link 333.
To access data sent by node 332, node 336 uses an add/drop filter tuned to a target frequency of 2xc2x71014 Hz. The filter removes electromagnetic radiation at or near a frequency of 2xc2x71014 Hz (the xe2x80x9cdroppedxe2x80x9d frequency) from link 333; all other surrounding frequencies are unaltered by the filter and node 336, and are placed on link 335. Node 336 may accept the data sent on the dropped frequency and transmit this data to, for example, customers serviced by network 300. Node 336 may add data to the frequency or alter the data on the frequency and use the filter to add this altered data stream, as a beam at frequency 2xc2x71014 Hz, to the beam for transmission on line 335. Frequencies other than the dropped frequency exist which may be filtered by the filter; however, such frequencies exist some distance away on the electromagnetic spectrum from the dropped frequency and are typically not included with the frequencies applied to the filter. Thus the filter accesses all frequencies surrounding the target band of frequencies; the target frequency is the frequency to which the filter is tuned.
It is sometimes desirable to reconfigure a network and reconfigure the frequencies that nodes are able to access by altering the frequencies added and dropped by filters. This may be desirable for a number of reasons. For example, in network 300, node 336, accessing a first frequency sent by node 332, may instead need to communicate with node 334 by accessing a second frequency, sent by node 334. Traffic patterns in a network may have to be rerouted due to, for example, the failure of a link or node, an increase in traffic, or the addition of equipment. This rerouting may require nodes to access different frequencies.
Optical add/drop filters typically are manufactured to operate on one frequency. Some existing optical add/drop filters have a capacity to have the frequencies on which they operate altered, and are thus considered xe2x80x9ctunablexe2x80x9d; however, such filters typically are not tunable over a wide variety of frequencies. Furthermore such filters may be expensive and inefficient, and may have a wider bandwidth than is desired.
FIG. 3 is a block diagram of a tunable filter. Referring to FIG. 3, tunable filter 370 alters its frequency by altering the angle at which the beam of light strikes the filter. Such a filter operates over a limited range of frequencies, is inefficient, and, as the angle of incidence increases, the amount of light energy lost to the filter increases and the bandwidth decreases. Tunable filter 370 comprises a mount 372, rotatable around an axis 373; a filter 374, adding and dropping signals at a certain frequency at a given angle of incidence; an in fiber 376, adding a signal on a single frequency; an in lens 378; a drop fiber 380, receiving a dropped signal on a single frequency; a drop lens 382; an input fiber 384, providing a multiple frequency beam as input to tunable filter 370; an input lens 386; an output fiber 390, accepting a multiple frequency optical beam altered by tunable filter 370; and an output lens 392. Lenses serve to focus the beam when the beam travels between the fiber and free space.
A beam is input to tunable filter 370 by input fiber 384 and strikes filter 374. The beam comprises multiple frequencies of electromagnetic radiation. Tunable filter 370 is designed so that, for a light beam striking filter 374 at an angle xcex8, a certain frequency of electromagnetic radiation (the tuned frequency) passes through filter 374 and frequencies surrounding the tuned frequency are reflected by filter 374. In such a manner one frequency, the tuned frequency, passes through filter 374 and mount 372 to be received by drop fiber 380; the signal received by drop fiber 380 is the dropped signal. Frequencies surrounding the tuned frequency output by in fiber 376 are reflected off filter 374 at angle xcex8 and are received by output fiber 390. In fiber 376 may output a light beam at the tuned frequency; such a light beam passes through filter 374 and becomes part of the beam accepted by output fiber 390. Drop fiber 380 and output fiber 390 are moved in proportion to the change in the angle. The frequency filtered by tunable filter 370 may be altered by rotating filter 374 around axis 373 to alter xcex8. As xcex8 increases, the amount of electromagnetic radiation absorbed by filter 374 increases, as does the bandwidth of filter 374. Such a decrease in the efficiency of tunable filter 370 and increase in the bandwidth of tunable filter 370 is not desirable. The range of frequencies to which the filter may be tuned is narrow. Furthermore, that fibers must be moved proportionally with the filter makes such a tunable filter difficult to implement.
FIG. 4 is a block diagram of a multiplexing/demultiplexing filter 400 for accessing multiple frequencies. Referring to FIG. 4, filter 400 includes clear holder 401, on which is mounted filters 402, 404, 406 and 408, each of which allows a band of frequencies to pass through and reflect all other frequencies; fibers 410, 412, 414, 416, 418 and 420, each either inputting a signal to or receiving a signal from multiplexing/demultiplexing filter 400, and each of which has attached one of lenses 422, 424, 426, 428, 430 and 432. Each of filters 402-408 filters a different frequency by allowing that frequency to pass through and reflecting other frequencies.
Multiplexing/demultiplexing filter 400 may act as a demultiplexer. In such a case a multiple frequency signal is input by fiber 410. At each of filters 402-408, one frequency is dropped and passes through the filter to one of fibers 412-420; the remaining frequencies are reflected to another of filters 402-408. Filter 408 reflects the last frequency to be dropped to fiber 420. Multiplexing/demultiplexing filter 400 may also act as a multiplexer. In such a case a signal on one frequency is input by each of fibers 412-420. Each of filters 402-408 allows the frequency input by its corresponding fiber to pass through the filter and to be combined with the multifrequency signal being generated; each such filter reflects all other frequencies in the multifrequency signal being generated. For example, filter 406 allows the frequency provided by fiber 416 to pass through filter 406; filter 406 reflects the frequencies provided to filter 406 by filter 408 and by fiber 418. In such a manner a multifrequency signal is generated and provided to fiber 410.
Multiplexing/demultiplexing filter 400 separates out multiple frequencies at the same time or combines multiple frequencies at the same time; such a system is expensive in that each frequency to be added or dropped requires its own fiber/lens/filter set. The expense of the equipment associated with each frequency to be added or dropped from multiplexing/demultiplexing filter 400 limits the number of frequencies which may be filtered. Furthermore, when demultiplexing, such a filter separates all frequencies on a fiber; it is often desirable only to access one of the multiple frequencies on a line. Thus, in such a system, the frequencies which are not to be removed must be recombined and placed back on the line.
Therefore, it is desirable to have a system which allows an optical filter to operate over a wide range of multiple frequencies, with a minimum of equipment costs, and a maximum of efficiency. It is desirable to have a tunable filter with a narrow bandwidth. Such a filter should be tunable easily and quickly, without the need to alter equipment or to physically assemble or disassemble equipment.
An inexpensive and efficient tunable electromagnetic filter is disclosed, having a wide range of tunable frequencies, comprising a holder with a number of filters mounted on the holder. The frequency of the filter is altered by moving the holder relative to a beam striking the holder so that one of the filters is filtering the beam. The frequency to be filtered may be easily and quickly changed, without altering the structure of the filter. In one embodiment, the tunable filter is a disk rotatable by a motor with a number of fixed frequency filters mounted around the periphery of the disk. The filter is surrounded by four fibers providing and receiving beams, as with known add-drop filters.